Natural killer (NK) cells constitute 15% of peripheral blood lymphocytes and play an important role in the ability of the innate immune system to fight off viral infections and also cancer (Purdy A K et al. (2009) Cancer Biol Ther 8:13-22). NK cells bind to target cells through multiple receptors, including natural cytotoxicity receptors (NCR), the Fc receptor CD16, NKG2D, and others. Binding of ligand to receptor initiates tyrosine phosphorylation and recruitment of accessory signaling molecules. This cascade results in activation of the NK cell, release of preformed granules containing perforin and granzymes into the target cell, and apoptosis. The concurrent release of cytokines and chemokines results in a micro-environmental milieu that recruits other immune cells.
NK cells have the capability of binding every cell in the body (Murphy W J et al. (2012) Biol Blood Marrow Transplant 18:S2-S7). However, binding of normal cells does not result in cytotoxic activity because of the ability of NK cells to simultaneously utilize a different set of receptors to bind major histocompatibility complex (MHC) class I molecules. Binding of human leukocyte antigen (HLA) E to the NKG2A/CD94 heterodimeric receptor, or of HLA-A, B and C molecules to inhibitory killer Ig-like receptors (KIRs), results in tyrosine phosphorylation, recruitment of the signaling adaptors SHP-1 or SHP-2, and downstream signaling. The end result is a dominant signal that suppresses normal activation signals. Thus, KIR/HLA interaction can impact NK cell responsiveness and also the development of the total number of mature responsive NK cells, known as licensing.
There are seven inhibitory KIRs and seven activating KIRs, which is one factor that results in diversity of KIR inheritance and expression. KIR is also expressed on natural killer T (NKT) cells and a small subset of T cells (Uhrberg M et al. (2001) J Immunol 166:3923-3932). Thus, mechanistically, blockade of inhibitory KIR could induce anti-tumor effects by allowing for activation of NK cell and possibly also some T cells.
Evidence in support of NK cell involvement in the anti-tumor response comes from the hematopoietic stem cell transplant (HSCT) setting. Given the diversity in both KIR and HLA, it is not surprising that KIR on donor NK cells may not interact with host HLA, referred to as KIR mismatch. The finding that AML patients transplanted with KIR mismatched donor NK cells had lower relapse rates (3% versus 47%, p<0.01) and reduced risk of relapse (relative risk 0.48, 95% CI 0.29-0.78) gave scientific support for the role of NK cells in the anti-tumor response (Ruggeri L et al. (2007) Blood 110:433-440).
In melanoma, certain KIR and HLA combinations may provide a more immunosuppressive environment, since certain combinations are seen more frequently in metastatic patients compared to non-metastatic patients (Naumova E et al. (2005) Cancer Immunol Immunother 54:172-178). KIR mismatch has been shown to be a favorable prognostic marker for high risk neuroblastoma patients undergoing autologous HSCT (Delgado D C et al. (2010) Cancer Res 70:9554-9561). Experimental support for the important role of NK cells in solid tumors comes from murine studies in which mice lacking T cells could still eradicate large solid tumors following NK cell activation by the addition of IL-15 (Liu R B et al. (2012) Cancer Res 72:1964-1974).
Full activation of naive T cells requires stimulation of the antigen receptor by peptide-major histocompatibility complexes and by co-stimulatory signals. These signals are provided by the engagement of CD28, which is constitutively expressed on T cell surfaces, with CD80 (B7.1) and CD86 (B7.2) molecules, which are present on antigen presenting cells (APCs). Cytotoxic T lymphocyte antigen 4 (CTLA-4; CD152) is an activation-induced T cell surface molecule that also binds to CD80 and CD86, but with greater avidity than CD28. CTLA-4 ligation down-regulates T cell responses, which results in abrogation of the effects provided by T cell activation. The blockade of CTLA-4 interaction with CD80/86 results in increased T cell activation. Ipilimumab (Yervoy®) is a fully human IgG1κ monoclonal antibody targeting CTLA-4 that inhibits the negative downstream signaling that occurs when CTLA-4 engages its ligands, CD80 and CD86, on APCs. As a result, activated T cells are able to maintain their CD28 mediated signaling resulting in IL-2 secretion and proliferation of CD8 T cells in response to an antigen.
Ipilimumab is currently approved for the treatment of metastatic melanoma, for which it has shown an overall survival advantage (Hodi F S et al. (2010) N Engl J Med 363:711-723). Administration of 4 doses of ipilimumab alone or in combination with a gp100 peptide vaccine improved survival by 4 months compared with the administration of gp100 vaccine alone in subjects with metastatic melanoma who had progressed after chemotherapy or IL-2 therapy.
Patients with metastatic or refractory tumors have very poor prognosis (Rosenberg S A et al. (2011) Cancer immunotherapy in Cancer: Principles & Practice of Oncology (Eds. DeVita V T, Lawrence T S and Rosenberg S A) 332-344 (Lippincott Williams & Wilkins, Philadelphia Pa.)). Despite advances in multimodal therapy, increases in overall survival in this patient population have been limited. Accordingly, it is an object of the present invention to provide improved methods for treating subjects with such tumors (e.g., advanced solid tumors).